Author:

Michael Quinsat(CEA Leti / Spintec)

Spin Torque Oscillators (STO) are nano-sized auto- oscillators whose
frequency can be tuned in a wide range. This tuning originates from the
nonlinear properties of the underlying magnetization dynamics that is
induced by spin transfer torque (STT) in magnetic nanostructures. Being
highly tunable in frequency has the inconvenience of being very sensitive to
noise. As a result the spectral purity of STOs is below the one required for
applications.
The magnetization dynamics induced by STT has been described theoretically
in the frame of a non-isochronous auto-oscillator model [1]. Within this
model the important information on the excitation mode is contained within
two phenomenological parameters which are, the amplitude-phase coupling $\nu
$, and the amplitude relaxation rate $\Gamma_{\mathrm{p}}$. They
completely determine the non-autonomous oscillator response and their
experimental determination is thus an important issue.
This presentation describes several experimental methods to extract these
parameters. The first involves time domain noise spectroscopy which permits
the power spectral density of phase and amplitude noise to be extracted [2].
The analysis of such noise in light of theoretical models allows not only
direct extraction of the nonlinear parameters, but also to quantify the
phase noise - the characteristics important for applications. This is
demonstrated for magnetic tunnel junction devices.
A second method involves the analysis of higher harmonic linewidths $\Delta
$f$_{\mathrm{n}}$ [3], where it is shown that due to the non-isochronous
property of STOs, the relationship between $\Delta $f$_{\mathrm{n}}$ and
$\Delta $f$_{\mathrm{1}}$ is nontrivial and allows one to extract $\nu $ and
$\Gamma_{\mathrm{p}}$.
We then apply the information gathered on the autonomous dynamics of STOs to
understand their non-autonomous dynamics that are a prerequisite for the use
of STOs in more complex devices (field sensors, frequency synthesizers,
etc). It is shown experimentally how the nonlinear parameters $\nu $ and
$\Gamma_{\mathrm{p}}$ determine this non-autonomous behavior of the STO.
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[1] A. Slavinand and V. Tiberkevich, IEEE Trans on Mag 45, 1875 (2009)\\[0pt]
[2] M. Quinsat et al. APL 97, 182507 (2010)\\[0pt]
[3] M. Quinsat et al. PRB 86, 104418 (2012)

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2013.MAR.F14.8